CAPE CANAVERAL, Fla. -- As much a symbol as it is a vehicle, the massive white spaceplane poised on the pad at Kennedy Space Center for one last launch has made the United States the world leader in space for three decades.

But soon the final space shuttle mission will end and the United States will have lost — at least temporarily — the ability to independently send astronauts into space. It comes at a time when other nations like China, India, perhaps even Iran, are entering the arena.

And while NASA has launched a constellation of futuristic satellites, exploratory robots and probes to the very edge of the solar system, when it comes to human spaceflight, no one is certain when the United States will be back.

“Everyone has known this was coming for years as the space shuttle program wound down,” said Paul Hill, NASA flight director for the space shuttle. “But now that we’re looking at the last shuttle mission, it’s dawning on people that this is it, and there’s nothing to replace it.”

U.S. astronauts won’t be entirely grounded, thanks to their former rivals for space supremacy. With the shuttle out of commission, the next American headed to the International Space Station, or ISS, in September will take a $50 million ride on one of Russia’s antiquated but reliable Soyuz rockets.

NASA has had gaps before when no human spaceflight program was up and running. But what’s different now, some department veterans and space analysts say, is that thanks to political wrangling centering on the role of promising but untested private space companies, NASA lacks a clear road map for putting people back in space.

“I was around in the ’70s when we canceled the Apollo program,” said Robert Crippen, a veteran of four shuttle flights who went on to direct the entire shuttle program from Washington. “But at least we were actively building the shuttle program and moving forward then. I think it’s hard for people to see what’s coming now.”

Retiring the shuttle

An explanation of how the United States came to this point starts with the limitations of the shuttle itself, a spacecraft that’s proved more expensive, complex and dangerous to fly than foreseen. At the program’s inception, NASA planned to ramp up to 50 or more launches a year, with the shuttle deploying and servicing commercial and military spy satellites and building space stations.

But in the wake of the breakup of Space Shuttle Columbia on re-entry in 2003, the program’s second fatal accident, after the 1986 Challenger explosion, President George Bush ordered NASA to phase out the shuttle once the ISS was built.

“I think the decision to stop flying the shuttle was a good one, the way it was made,” said Eileen Collins, a veteran of four shuttle flights and commander of the first mission after the Columbia disaster. “The plan was to design and build a replacement for the shuttle that would eliminate those inherent design flaws.”

Ditching the radical airplane-style layout of the shuttle, NASA’s planned replacement was a traditional-looking rocket topped by a crew capsule, with a maiden flight scheduled in 2014 after a four-year hiatus from human spaceflight. But with authorization from Congress and encouragement from the Bush administration, the agency was aiming even higher.

The new human spaceflight program, dubbed Constellation, also included a giant heavy-lift rocket and lunar spacecraft for a planned return to the moon by 2020. The moon, in turn, would serve as a springboard, Bush said: “Establishing an extended human presence on the moon could vastly reduce the cost of further space exploration, making possible ever more ambitious missions.”

With the NASA authorization bills of 2005 and 2008 providing a clear direction, NASA was embarking on its next agency-defining space program in a line that stretches from Mercury and Gemini to Apollo and the space shuttle.

“You had a consensus on space that was bipartisan, bicameral — something extremely rare in Washington,” said Eric Sterner, an analyst for the conservative George C. Marshall Institute who served as NASA deputy administrator of policy and in the Bush administration. “And the current administration came in and blew it up.”

New direction

Just over a year into his first term, President Barack Obama told NASA to refocus its human spaceflight efforts on distant exploration and drop the idea of building a manned vehicle to replace the space shuttle in low-Earth orbit.

Political consensus or no, the Constellation program was a loser, said the White House Office of Management and Budget — “over budget, behind schedule and lacking in innovation due to a failure to invest in critical new technologies.”

The Augustine Commission, appointed by Obama to study national space policy, had reported in late 2009 that Constellation’s Ares 1 rocket would miss the 2014 launch goal by years. Meanwhile the program was spending billions of dollars in the midst of a serious financial downturn.

In a major shift in U.S. space policy, Obama declared that returning the United States to low-Earth orbit would be an effort led by aerospace companies — ranging from entrepreneur Elon Musk’s promising upstart SpaceX to established players including Orbital Sciences and Boeing — and not NASA.

“In order to reach the space station, we will work with a growing array of private companies competing to make getting to space easier and more affordable,” Obama said in an April 2010 speech at Kennedy Space Center.

Obama in the same speech promised NASA billions to fund research on a heavy lift rocket for deep-space exploration, and said that rather than following Bush’s vision of returning to the moon by 2020, NASA would aim to land astronauts on an asteroid. Later, by the mid-2030s, astronauts would orbit Mars.

Low-Earth orbit, an orbital zone a few hundred miles above the surface traversed by the space shuttle and other craft, essentially is a “been there, done that” proposition, say supporters of Obama’s plan.

“It’s really a sign of optimism and a belief that the future of government space programs lies in exploration far beyond Earth,” said John Logsdon, professor emeritus at George Washington University and former director of the Space Policy Institute. “All we’re talking about with (commercial spaceflight) is a taxi service to the space station.”

John Shannon, NASA’s current space shuttle program manager, says that if private firms manage to take over the quotidian taxi duty, “that frees up NASA to do something we haven’t done since Apollo — go outside low-Earth orbit.

“We’re talking about going back into lunar orbit, to the moon, near Earth asteroid activities and maybe eventually to Mars,” Shannon said. “It’s a pretty exciting time.”

Private risk

But if the private firms Obama is betting on fail, what then?

Depending on commercial firms for access to the ISS puts the success of the orbiting science lab at risk, said Scott Pace, current director of Space Policy Institute at George Washington University.

“I would have preferred a more diversified portfolio with a government system as a base, and then bringing in commercial firms,” Pace said. “If those firms have problems or delays, the government system can be a backup.”

There’s another risk, he said: The potentially crushing cost of exploring distant locations like Mars might require the United States to work closely with partner nations as it has on the International Space Station. But potentially leaving the ISS in the lurch by not having a government space taxi available if private firms fail could damage other nation’s trust in the U.S. commitment to space enough to hamper such cooperation.

“The current administration’s proposal has unintentionally created barriers to working with internationals on future human space exploration,” Pace said.

Future destinations

Obama’s proposals for future U.S. human space exploration “ran into a buzz saw in Congress,” said Sterner of the Marshall Institute. Legislators in both parties, many with constituencies that depended on Constellation contracts for jobs, refused to relinquish the program.

NASA is trying to thread the needle between Obama’s and Congress’ differing visions, and is readying a proposal on a heavy lift rocket that remains unfunded. But Sterner worries that in the wake of the impasse, space exploration could get shoved to the back burner by more pressing political priorities.

“Whether or not human space flight is still in NASA’s future I believe is still an open question,” he said.

“It’s not because anyone wants it to end, but because maybe we can’t get our crap together in a reasonable time frame to make sure it doesn’t.”

Such doubts are being raised more frequently in Washington as the shuttle program nears the end, and they brought a strong reaction from NASA’s administrator, retired Marine Corps Maj. Gen. Charles Bolden, in a Washington speech last week.

NASA is working through a complex period of setting new goals and marshaling resources, but coming decades will see astronauts exploring where people have never gone before, the four-time shuttle veteran said.

“So when I hear people say — or listen to media reports — that the final shuttle flight marks the end of U.S. human spaceflight, I have to say these folks must be living on another planet,” Bolden said. “The debate is not if we will explore, but how we’ll do it... NASA’s 21st Century mission will focus on the transportation systems that will carry us beyond where we have been, to new destinations and new milestones in the annals of human history.”